1. Field of the Invention
This invention relates to a permanent magnet made from a magnetic material consisting mainly of iron, and having an improved level of rustproofness, and a process for manufacturing the same. More particularly, it is concerned with a resin-bonded (hereinafter referred to simply as "bonded"), or sintered magnet composed of a rare earth-iron-boron ("Nd-Fe-B") alloy or compound, and a process for manufacturing the same.
2. Description of the Prior Art
It has long been known that there are alloys or compounds consisting mainly of iron, i.e. containing at least 50 atom % of iron, and exhibiting very high magnetic properties, since iron is an element having a higher saturation magnetic flux density at room temperature than that of any other element, and that those alloys or compounds can be used to make, for example, resin-bonded or sintered permanent magnets having very high magnetic properties. Nd.sub.2 Fe.sub.14 B, SmFe.sub.12 and Fe.sub.16 N.sub.2 are examples of recently developed alloys or compounds exhibiting very high magnetic properties. These alloys or compounds, however, have the drawback of being easily oxidized to get rusty, since they contain a high proportion of iron. This is particularly the case with Nd-Fe-B magnets for which there has recently been a growing demand. They easily get rusty in a highly humid environment. Various methods have, therefore, been proposed for rustproofing those magnets. They include coating the surface of a resin-bonded Nd-Fe-B magnet with an acrylic or epoxy resin (Japanese Patent Application Laid-Open No. 244710/1988 or 244711/1988), or with a fluorine-containing resin (Japanese Patent Application Laid-Open No. 168221/1986). There have also been made attempts to form an electrodeposited layer on the surface of a magnet, or plate it with a metal such as nickel.
All of the proposed methods, however, have their own drawbacks. The resin coating of the magnet surface is an incomplete rustproofing method, since it is difficult for the resin to shut off oxygen and water completely, though it is an economical method. Electrodeposition is a method which is economically unacceptable. Metal plating is also economically unacceptable and has, moreover, the drawback that a trace of plating solution remaining on the magnet surface may rather accelerate its corrosion.
Sintered Nd-Fe-B magnets are also very likely to get rusty in a humid environment, and are, therefore, plated with e.g. nickel. The drawbacks of such plating have, however, been already pointed out. The addition of chromium or nickel to the magnet material improves its corrosion resistance to some extent, but is not common practice, since it lowers the magnetic properties of the magnet.
Thus, all of the known methods for rustproofing a magnet composed of a rare earth alloy or compound, particularly Nd-Fe-B, are more or less defective, whether the magnet may be a bonded or sintered one.